- linking zmq fails under clang - `json.hpp` somehow isn't happy with gcc 4.9 (see also: nlohmann/json/issues/212, nlohmann/json/pull/1257)
json.hppsomehow isn't happy with gcc 4.9 (see also: support gcc 4.8.1 nlohmann/json#212, GCC 48 nlohmann/json#1257)